1. Field of the Invention
The present invention relates generally to a method of providing a group call service, and in particular, to a method of providing an extended group call service that allows a subscriber to a predetermined group call service to receive the group call service from a neighboring group call area if he is located out of a group call service area, in a radio network environment supporting group call service.
2. Description of the Related Art
In general, GSM (Global System for Mobile communication) supports group call service. A group call service refers to concurrent provisioning of the same information to a plurality of subscribers to the group call service via one channel. Besides a paging channel (PCH), a base station (BS) that services a group call broadcasts a group call channel (GCC) to mobile stations (MSs) within its service area.
FIG. 1 illustrates a typical group call service environment. A group call service-supporting BS BS1 is adjacent to a non-group call service-supporting BS BS2 in FIG. 1. Referring to FIG. 1, BS1 supports a group call service and its service area 10 is called a group call area (GCA). BS2 does not support the group call service and its service area 20 is called a Not_GCA.
FIG. 2 illustrates an example of information that a typical group call service-supporting BS broadcasts to MSs within its service area. In FIG. 2, reference numeral 30 denotes a general PCH, while reference numeral 40 denotes a GCC that provides basic information about the group call service.
Referring to FIG. 2, the general PCH30 includes BS_ID, the ID of a BS that has generated the PCH, and general paging information (e.g. the received signal strength of the BS and position information). The GCC40 includes BS_ID, the ID of a BS that has generated a notification channel (NCH), and group call paging information. The group call service-supporting BS broadcasts both the general PCH30 and the GCC40. Meanwhile, the non-group call service-supporting BS broadcasts only the general PCH30. Thus, subscribers to the group call service receive both the PCH30 and GCC40.
In the case illustrated in FIG. 1, BS1 broadcasts both the general PCH30 and GCC40 as illustrated in FIG. 2 because it provides the group call service. On the other hand, BS2, which does not provide the group call service, broadcasts only the general PCH30. Therefore, MS1 within the GCA 10 receives both the general PCH30 and GCC40, whereas MS2 within the Not_GCA 20 receives only the general PCH30.
Meanwhile, MS3 is located in an overlap area between BS1 and BS2, so that it can receive information from both BS1 and BS2. MS3 compares the received signal strengths of PCHs from BS1 and BS2 and registers to a BS having the higher received signal strength. In the illustrated case of FIG. 1, MS3 receives higher signal strength from BS2 and so registers to BS2, thereby receiving a radio communication service from BS2. Thus, MS1 receives a radio communication service from BS1, while MS2 and MS3 receive radio communication services from BS2.
If MS3 is a subscriber to the group call service from BS1, it cannot receive the group call service due to its registration to BS2 even though it is positioned where data can be received from BS1.
Traditionally, even when a subscriber to a particular group call service is within the service area of a GCA of the group call service with received signal strength enough to stably receive the group call service, but the subscriber has registered to another BS that offers a higher received signal strength than a BS that provides the group call service, but not a GCA of the group service call service, and consequently, the subscriber cannot receive the group call service.